Gold colored metallic pigments



United States Patent This invention relates to the preparation ofgold-colored metallic pigments. More particularly, it relates toaluminum powders which have been treated to produce a gold-colored filmon the surface of the particles while retaining the characteristicmetallic luster of the aluminum powder.

It has already been proposed to produce colored aluminum powders byapplying organic dyestuffs to oxidecoated aluminum powders. It has alsobeen proposed to produce colored aluminum by a multi-stage processwhereby the aluminum is first treated to produce a coating thereon ofaluminum oxide, then the so coated material is soaked in a solutioncontaining an oxidizable metal ion, and finally the oxide-coatedaluminum with the oxidizable metal ion absorbed on and in the oxidecoating is treated to oxidize the oxidizable metal ion. Also, severalprocesses have been proposed for the treatment of aluminum to producedark, electrically non-conductive and corrosion-resistant surfaces.

None of these prior art methods, however, has been satisfactory for theproduction of gold pigments suitable for decorative inks and enamels.When organic dyestuffs are employed, the products may be made withexcellent initial color, but lack lightfastness, and may bleed whenapplied in an ink. Other chemical coatings on aluminum powders have beendark and lacking in the desired metallic luster. To some extentcopper-containing alloys of aluminum have been used when a gold-coloredmetallic pigment has been required, but these powders besides lackingthe desired rich luster and color of gold are also subject to tarnishingso that inks made therefrom lack color stability both in storage andafter application.

Accordingly it is an object of this invention to provide an aluminumpigment which is lightfast, of good storage stability, and inert to thecommon ink and lacquer ingredients. Another object is to provide amethod of preparing such a pigment, which method is simple andeconomical to carry out. Other objects will become apparent from thedetailed description which follows.

The novel pigments of this invention may be prepared by agitatingaluminum powder with a 0.1 to 0.4 molar aqueous solution of an alkalimetal permanganate, which treatment produces a coating on the particlesof aluminum such as to give the powder a rich golden appearance.Preferably the solution is acidulated to give an initial pH of about 3.Certain other additives and modifiers are preferably added to thesolution in order to control its reactivity and to improve the lightstability of the finished pigment. The redder golds may be moreconveniently formed by adding small arnounts of oXidizable substances,such as formaldehyde, acetaldehyde, methanol, ethanol, alkanolamines,n-propanol, isopropanol, ethylene glycol, and other substances. Thesesubstances, hereinafter are referred to as reductants.

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The process is applicable to commercial dry aluminum powders of theleafing as well as the non-leafing type, having mesh sizes coarser than425. Aluminum powders of mesh finer than about 400 have provideddifiicult to treat by the process of the present invention. Powder of325 mesh has been found very suitable, as have coarser powders also. Ithas even been found that large pieces of aluminum, such as sheets, rods,cast shapes, etc., can also be treated to provide a gold-colored coatingby use of our process.

Best results have been obtained when using potassium permanganatesolutions of about 6% concentration. More dilute solutions have beensuccessfully used, down to about 0.1 molar permanganate concentration,but for the deeper colors the higher concentrations should be used.Other alkali metal permanganates are suitable, but We prefer potassiumpermanganate for economy and convenience. The process of the presentinvention is operative over a temperature range from 5 C. to 0,depending on the time of reaction and the particular composition of thesolution used. Processing time in the: coloring bath usually varies fromhalf an hour to siX hours.

The gold color is visible in 1 minute.

Weight ratios of treating solution to aluminum between 10 and 30 arepreferred. When the ratio is less than 10 the time of treatment becomestoo long for convenience and proper agitation may be more difficult.With ratios higher than 30 the time of treatment becomes so short thatstopping the color development at the stage desired is impractical.

A gold powder having a metallic luster can be obtained in the absence ofa reductant but in the presence of a reductant the process yieldsdarker, deeper shades. Particularly suitable reductants are loweraliphatic alcohols including glycols, lower aliphatic aldehydes, andalkanolamines although various other reducing substances may be used. Tosome degree the reductant chosen will affect the color obtained, forexample methanol and the ethanolamines produce reddish gold toneswhereas formaldehyde, ethanol, and propanol favor the formation ofgreenishyellow tones.

Air drying and oven drying up to 100 C. give identical products, but thelatter is preferable because better control is possible. Heating thepigment at temperatures between 100 and 200 degrees C. intensifies thecolor but reduces the lightfastness.

To improve the dispersing properties of the finished product, the bestresults are obtained by giving the pigment a final wash with stearicacid in methanol before drying, or by hand milling the dry powder withstearic acid. Also passing the dry pigment through a 325- mesh screenimparts very satisfactory dispersing proper ties to the dry pigment.

The acid used in my preferred procedures should be of such strength andconcentration that the pH of the solution will remain substantiallywithin the limits of 2 and 9.5 during the course of the process.

To throw some light on the chemistry of our process, a brief review ofsome pertinent information from the literature inserted here may beuseful.

Aluminum, and in particular aluminum powder, has a strong tendency todecompose water, with evolution of hydrogen, and in solutions which donot form complexes with aluminum ions, the metal tends to acquire aprotective oxide film. The exact nature of this oxide is not known,although it is believed the inner portion near the metal is oxide, whilethe outer portion is hydrated oxide or can be hydrated by sufficientcontact with aqueous solutions. in a solution free of substances whichare able to form soluble complexes or insoluble salts, corrosion takesplace below a pl-I of 4 and above a pH of 3.6. In actual practicethecorrosion behavior of aluminum is determined by the behavior of thefilm of oxide, with which it is always covered, towards the solutions incontact with it. In general, the film of oxide dissolves slower inacidic solutions than in alkaline ones, but it is not soluble inammonia. Water alone usually does not attack it.

An oxide coating may be deposited on aluminum by immersing the metal ina solution where it will be attacked to a limited extent by oneingredient and simultaneously oxidized by another. Such a solution isused, for example in the Alrok treatment where aluminum is immersed in ahot solution of sodium carbonate and sodium dichromate. The filmformation depends upon a definite balance between the action of the twocomponents. The rate of growth of the film depends to a large extentupon the rate of diffusion through it.

Iydrogen gas reduces solutions of alkali metal permanganates, especiallywhen warmed. It has been found that a well stirred solution of, forexample, 5% potassium permanganate absorbs hydrogen gas at a steadyrate. In acidic solutions hydrogen precipitates finely-divided MnO andoxygen is evolved. In approximately neutral solutions the reactionproduces colloidal manganese dioxide. No oxygen is evolved but KOH isformed so that the reaction medium quickly becomes alkaline unless theKOH can be disposed of. In alkaline solution reduction by hydrogen isslow, K MnO and MnO are first formed. Later K MnO breaks down to M110and KOH.

The oxidizing action of alkali metal permanganates is most energetic inacidic solutions, two molecules of permanganate yielding five atoms ofoxygen, whereas in alkaline solutions only one atom of oxygen would bereleased. Neutral solutions produce three atoms of oxygen from twomolecules of alkali metal permanganate but, since alkali metal hydroxideis a byproduct of this action, this solution would quickly becomealkaline unless the alkali was neutralized or removed.

The oxides of manganese produced from permanganates by the action ofgaseous reducing agents are more finely divided and are capable ofreacting more rapidly than those produced by a solid or liquidreductant.

In acidic solutions there is an initial incubation period beforepermanganate solutions yield their full power of oxidation, but thisdelay may be considerably shortened when a reducing agent is present.

With these and other considerations in mind, we believe our processoperates by gradually co-depositing finely divided, perhaps colloidal,oxides of manganese and the type of aluminum oxide that is adherent,transparent, and tough onto the bright surfaces of the aluminum metal insuch a way that the metallic luster is still plainly evident and thepigment as finally processed has acquired a color ranging from greenishyellow through golden yellow, reddish gold, and somewhat browner orgrayer hues, the color depending upon the conditions chosen. Themanganese dioxide produced outside this coating sometimes aids in givingthe effect of a deep shade. Action begins when the surfactant hasremoved the coating of oil or other similar material from the aluminummetal. In acidic processing baths the initial pH is preferably about 3.As the reaction proceeds potassium acetate is formed, quickly raisingthe pH to the range of about 4-6, where it remains in those examples ofour process that operate entirely on the acid side, like Example No. 1.Such pigments acquire considerably more lightfastness when aftertreatedwith a dilute alkali such as sodium or potassium hydroxides orcarbonates. Ammonium hydroxide will not do. When the processing bathreaches a plateau in the mildly alkaline region, pH 7.1-9.5 as inExamples 2 and 8, alter-treatment with alkali is not necessary forimproving the lightfastness. Methanol is oxidized mainly to carbondioxide at low pH values but increasingly to formic acid in theneighborhood of pH 6-7 and on the alkaline side. The pH of the methanoltreated baths levels off after awhile around pH 8-9. This isapproximately the pH Value of potassium acetate and formate solutions.The rise in pH is facilitated by the oxidation of methanol or othereasily oxidizable compounds that we prefer to add, because of theformation of additional potassium acetate, potassium formate or similarsalts, as well as by the depletion of the acid originally present. Whenthe solution is more or less neutral the manganese acetate or similarmanganese salt, formed Well on the acid side of neutrality, reacts withpotassium permanganate to produce manganese oxides and potassium acetateor similar potassium salt.

For the operation of our invention in strongly alkaline solutions, thealkali should be one that does not attack the oxide film on thealuminum, ammonium hydroxide being the alkali. we prefer. Hydrogen gasis vigorously evolved at the start of the process and gradually reducessome KMnO; to K MnO, and oxides of manganese. After a while K MnQ,breaks down to MnO and KOH. Through the reaction 2Al-l-6NH OH=2Al(OI-I)+I-I +2NH ammonia is gradually lost from the solution by evolution as agas mixed with hydrogen. The pH then decreases from its initial value ofabout 11. At the point where appreciable amounts of KOH are generated bythe breakdown of 1 1 124110 the treatment apparently must be stoppedbecause KOH would tend to attack the oxide coating on the aluminum,especially if the ammonium radical in solution is much depleted. Thecoatings produced by this highly alkaline bath process are, however, notquite as desirable as those produced by my other solutions.

The following examples are given as illustrations but not aslimitations:

Example 1 300 grams of 325-mesh aluminum powder are gradually added to asolution, at room temperature, of 379.2 grams potassium permanganate, 72grams glacial acetic acid, and 120 milliliters of a 3.5% solution inwater of sodium lauryl sulfate dissolved in 6 liters of water. When thealuminum powder has been added and wetted, 36 milliliters of methylalcohol are added dropwise. The solution is stirred vigorouslythroughout the process. The temperature rises about 10. 45 minutes afterall the alcohol has been added the slurry is filtered, the pigment iswashed with water, then washed with acetone, and oven dried at 60 C. forone hour. 250 grams of this product is slurried in 7.5 liters of 0.025%sodium hydroxide solution for 15 minutes, filtered, washed free ofalkali with water, washed with acetone, and oven dried at 60 C. for onehour. Its dispersing properties can be improved by screening through a325-mesh screen.

Example 2 100 grams of 325-mesh aluminum powder are gradually added to asolution, at about room temperature, of grams potassium permanganate, 12grams glacial acetic acid, and 20 milliliters of a 3.5% solution ofsodium lauryl sulfate in 1500 milliliters water. When all the aluminumpowder is wet, 24 milliliters of methanol are added dropwise. Thesolution is stirred vigorously for 5 or 6 hours, if a dark reddish goldis wanted. The temperature should be kept under 40 C. If a lighter,yellow gold is desired, the reaction should be stopped after about 2 to4 hours. When the reaction has produced the color wanted, the slurry isfiltered, the pigment washed free of permanganate with water, washedwith acetone, and oven dried at 60 C.

Example 3 Add, with constant stirring, grams of 325-mesh aluminum powderto a solution at room temperature of 126.4 grams potassium permanganate,24 grams glacial acetic acid, and 40 milliliters of a 3.5% solution ofsodium lauryl sulfate in 2000 milliliters of water. When all thealuminum powder is wetted, 6 milliliters of ethylene glycol is addeddropwise and the solution stirred 45 minutes. The slurry is washed freeof permanganate with water, washed with acetone, and oven dried at 60 C.for one hour. 134 grams of the resulting pigment is slurried minutes in4 liters of a 0.025% sodium hydroxide solution, filtered, washed free ofalkali with water, washed with acetone, and oven dried at 60 C. for onehour.

Example 4 100 grams of 325-mesh aluminum powder is added with constantstirring to a solution at room temperature of 126.4 grams potassiumpermanganate, 24- grams glacial acetic acid, and 40 milliliters of a3.5% sodium lauryl sulfate solution in 2000 milliliters of water. Whenall the powder has been wetted, add 12 milliliters of a solution oftriethanolamine titanate and isopropanol. Stirring is continued for onehour. The slurry is then filtered, washed with water, washed withacetone, and oven dried at 60 C. 130 grams of the resulting pigment areslurried 5 minutes in 4 liters of 0.025% sodium hydroxide solution,washed free of alkali, washed with acetone, and oven dried at 60 C.improved dispersing qualities can be imparted to the pigment at thisstate by shaking 30 grams in a quickie mill with a mixture of 0.9 gramstearic acid, 0.3 gram sodium lauryl sulfate, 0.02 gram sodiumhexamethaphosphate, and milliliter of water. The shaking is done in 3periods of 5 minutes each spaced by cooling periods of 25 minutes. It isfinally oven dried at 60 C.

Example 5 50 grams of 325-mesh aluminum powder is added with constantstirring to a solution at room temperature of 63.2 grams potassiumpermanganate, 12 grams glacial acetic acid, and 20 milliliters of a 3.5%solution or" sodium lauryl sulfate in 1 liter of water. Stirring iscontinued for 1 or 2 hours to give a red gold pigment. Darker shades maybe obtained by continuing the process 4 to 5 hours. When the desiredresult is obtained, the slurry is filtered, washed with water, then withacetone, and oven dried at 60 C. For runs of two hours and less thelightfastness can be further increased by the after treatment withdilute caustic solution as in Examples 1, 3, and

Example 6 150 grams of 325-mesh aluminum powder is added with constantstirring to a solution at room temperature of 189.6 grams potassiumpermanganate, 45 milliliters of concentrated ammonium hydroxide, andmilliliters of a 3.5% solution of sodium lauryl sulfate in 3 liters ofwater. Stirring is continued minutes, the temperature being kept below40 C. The slurry is filtered, washed with water, then washed withacetone, and oven dried at C.

It is to be understood that my invention is not limited to the specificdetails herein described since these can be varied without departurefrom the spirit of the invention.

Example 7 In gredients:

50 gms. 100-mesh, leaiing aluminum powder 63.2 gms. KMnO, in 1 literdistilled water 50 mls. 3.5% sodium lauryl sulfate solution 11.5 mls.glacial acetic acid 6 mls. methanol The procedure of Example 1 isfollowed.

Example 8 110 gms. KMnO are dissolved in 1.75 liters of water and cooledto 25 C. To this are added 20 mls. of 3.5% of sodium lauryl sulfate and18 mls. of glacial acetic acid. 100 gms. of 325-mesh aluminum powder arenow added 6 with vigorous stirring. After all the powder is in, thetemperature is lowered to 25 C. and 18 mls. of methanol added dropwise.Vigorous stirring is maintained for 6 hours. The temperature should riseto 3.5 C., but no higher, for one half to three quarters of an hourafter the methanol has all been added. Cool if necessary. The pigment iswashed and dried as in Example 1, but is not after-treated with alkali.The processing bath has a pl-l of about 8.08.8 during the last half ofthe treatment. The lightfastness is better than 500 hours.

We claim:

1. A pigment ranging in color from greenish yellow to dark reddish goldconsisting of pigment-sized particles of metallic aluminum coated with atransparent film of co-deposited oxides of manganese and aluminum.

2. A pigment ranging in color from greenish gold through yellow gold todark reddish gold consisting of pigment-sized particles of metallicaluminum coated with an adherent film of co-deposited oxides ofmanganese and aluminum, which film is sufficiently transparent toperunit the metallic luster of the aluminum to be visible.

3. The pigment of claim 2 in combination with that oxide of manganeseproduced in the process which is not in the film of co-deposited oxideson the aluminum, the combination having a metallic luster,

4. The pigment of claim 3 having a lightfastness of to 600 hour understandard Fadeometer conditions.

5. The pigment of claim 3 having a mesh size of 100 to 400.

6. A process of producing a goldcolored aluminum pigment which comprisesslurrying aluminum powder, coarser than 425 mesh, with a 0.1 molar tosaturated at about room temperature solution of alkali metalpermanganate, agitating the resulting mixture at a temperature between 5and 100 C. for a period of time sufficient to develop a gold-coloredsurface coating on the metallic particles, and separating the resultingpigment from the treating solution.

7. A method of producing a gold-colored metallic pigment which comprises(1) stirring aluminum powder, coarser than 425 mesh, in an aqueoussolution containing about 6% potassium permanganate and between 0.1% and3% acetic acid at a temperature between 5 and 100 C. for a period oftime between 1 minute and 8 hours (2) separating the thus treatedaluminum pigment from the aqueous solution and drying it promptly (3)contacting said aluminum pigment with a 0.1% to 0.005% solution ofsodium hydroxide (4) washing said aluminum powder with water untilsubstantially free of alkali, and (5) drying the resulting pigment.

8. A method of producing a gold-colored metallic pigment which comprises(1) stirring aluminum powder, coarser than 425 mesh, in an aqueoussolution containing about 6% potassium permanganate and between 0.1% and3% acetic acid at a temperature between 5 and 100 C. in the presence ofan organic substance which is substantially oxidized by the solution (2)stirring the solution for a period of time between 1 minute and 8 hours(3) separating the thus treated aluminum from the aqueous solution anddrying it promptly (4) contacting said aluminum pigment with a 0.1% to0.005% solution of sodium hydroxide (5) washing said aluminum powderwith water until substantially free of alkali, and (6) drying theresulting pigment.

9. A method of producing a gold-colored metallic pigment which comprises(1) stirring aluminum powder, coarser than 425 mesh, in an aqueoussolution containing about 6% of an alkali metal permanganate, a wettingagent, and between 0.1% and 3% acetic acid at a temperature between 5and 100 C. (2) adding an organic substance which is substantiallyoxidized by the solution and which is selected from the group consistingof lower aliphatic alcohols, glycols, lower aliphatic aldehydes, andalkanolamines, stirring the solution for a period of time between 1minute and 8 hours (3) separating the thus treated aluminum pigment fromthe aqueous solution, Washing it substantially free of permanganatepromptly, and (4) drying the resulting pigment.

10. The method of claim 9 wherein the-substance of step (2) is a loweraliphatic alcohol.

11. The method of claim 9 wherein the substance of step (2) is a loweraliphatic aldehyde.

12. The method of claim 9 wherein the substance of step (2) is a glycol.

13. The method of claim 9 wherein the substance of step (2) is analkanolamine.

14. A method of producing a gold-colored metallic pigment whichcomprises (1) stirring aluminum powder, coarser than 425 mesh, in anaqueous solution containing about 6% potassium permanganate, a Wettingagent, and 0.1 molar to 0.5 molar ammonium hydroxide at a temperaturebetween 5 and 50 C. for a period of time between 1 minute and .2 hours(2) separating the thus treated pigment from the aqueous solutionpromptly and washing it substantiallytree of alkali, and (3) drying theresulting pigment.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PIGMENT RANGING IN COLOR FROM GREENISH YELLOW TO DARK REDDISH GOLDCONSISTING OF PIGMENT-SIZED PARTICLES OF METALLIC ALUMINUM COATED WITH ATRANSPARENT FILM OF CO-DEPOSITED OXIDES OF MANGANESE AND ALUMINUM.
 6. APORCESS OF PRODUCING A GOLD-COLORED ALUMINUM PIGMENT WHICH COMPRISESSLURRYING ALUMINUM POWDER, COARSER THAN 425 MESH, WITH A 0.1 MOLAR TOSATURATED AT ABOUT ROOM TEMPERATURE SOLUTION OF ALKALI METALPERMANGANATE, AGITATING THE RESULTING MIXTURE AT A TEMPERATURE TUREBETWEEN 5* AND 100*C. FOR A PERIOD OF TIME SUFFICIENT TO DEVELOP AGOLD-COLORED SURFACE COATING ON THE METALLIC PARTICLES, AND SEPARATINGTHE RESULTING PIGMENT FROM THE TREATING SOLUTION.